Your image is stored in the PNG file as unsigned 16-bit integers. When imshow displays a uint16 image, it uses the unsigned 16-bit range by default: 0 is black, and 65535 is white.
Your image has one pixel with the value 32666 near the upper right corner, and it appears that all the other pixel values are less than 3000. In fact, almost all of the pixels equal 2843. When viewed on a [0,65535] scale, these values are almost black. Try setting the display range explicitly when you call imshow:
Here's what that looks like: